Warmer and Smaller

The roundtail chub is another fish that has been found to be shrinking in size. Photo Credit: National Park Service

Big fish that move fast could lose a bit of their swagger in the coming decades. Warmer ocean waters will provide less oxygen, which could reduce the maximum size of some species of fish -- especially those that are the most active.

Researchers have noticed that some species are already getting smaller -- species like herring, sole, and haddock. In part, that could be the result of overfishing -- the bigger fish get caught, leaving the smaller ones behind. But a recent study suggests that warmer oceans could also be playing a role.

Researchers reviewed more than 300 studies on 74 species of fish found in the Mediterranean Sea. The studies covered regions of different temperatures. The results showed that as the temperature went up, the maximum body size for any given species went down.

On average, size dropped by about five percent for every couple of degrees increase in temperature. But for more active species -- the ones that chase prey at high speeds -- the rate doubled -- a ten percent drop in body size for the same change in temperature.

The researchers say the amount of oxygen a fish consumes depends on the area of its gills. But the area of the gills grows at a slower rate than the fish’s overall size. So with less oxygen in the water, a fish might stop growing earlier than it would otherwise.

Smaller, slower fish might not be able to survive as well, which could reduce populations, or force some species to seek cooler, oxygen-rich waters -- a protection against the warming oceans.